Photovoltaic cells ("solar cells") are currently under intensive development for use as alternative energy sources. Much of this work has been driven by the urgency to reduce the dependency upon fossil fuels and the need for efficient energy production in space.
Silicon is the favored element for most photovoltaic cells, due in part to its relative abundance.
Hydrogenated amorphous silicon (designated "a-Si:H") has intrinsic semiconductor properties. Hydrogenation is believed to facilitate termination of "dangling bonds" in amorphous silicon. Hydrogenated amorphous silicon can be alloyed with other elements to form semiconductor materials (amorphous silicon "alloys") having a variety of optical bandgap energies, wherein the optical bandgap energy determines what wavelength(s) of light will cause the semiconductor to generate a photovoltaic current. Hydrogenated amorphous silicon can also be doped. Production of "device quality" amorphous silicon solar-cell material generally involves hydrogenation to some extent. "Device quality" is a term used imprecisely by persons skilled in the art but can be considered descriptive of material that works particularly well and efficiently in a solar cell.
Amorphous silicon can be "hydrogen diluted" wherein hydrogen is incorporated during processing into the amorphous atomic network of silicon. Hydrogen dilution is one way to change the optical bandgap energy of silicon.
Unfortunately, amorphous silicon solar cells are prone to a phenomenon in which they experience a significant reduction in power output (i.e., a reduction in conversion efficiency) after the cells have been exposed to light. This light-induced degradation is termed "Staebler-Wronski degradation" after the individuals who first observed and reported the phenomenon. Staebler and Wronski, Appl. Phys. Lett. 31:292 (1977). Although agreement has not been reached on the cause(s) of Staebler-Wronski degradation, the phenomenon appears to be a manifestation of an increase in the density of "defects" in the amorphous network upon prolonged exposure to light.
Staebler-Wronski degradation appears to be more or less self-limiting. Accelerated light soaking of a-Si:H and a-Si:H,F films using a krypton ion laser indicated that the light-induced defect density of these materials eventually reaches a saturated value. Park et al., Appl. Phys. Lett. 55:2658 (1989); Park et al., Appl. Phys. Lett. 57:1440 (1990); Redfield and Bube, Appl. Phys. Lett. 54:1037 (1989). Staebler-Wronski degradation is associated with an increase in defect density, but the mechanism by which the defect density changes during light soaking has not been elucidated. The general state of the art indicates that "device quality" materials have saturated defect densities of about 2.times.10.sup.16 cm.sup.-3 or less.
In one study of the saturation behavior of a number of a-Si:H films, the lower limit of saturated defect density appeared to be related to the optical bandgap energy of the material. Isomura et al., Solar Cells 30:177 (1991). The authors suggested that, by lowering the optical bandgap energy of a-Si:H, the saturated defect density could also be reduced. The optical bandgap energies were lowered by reducing the concentration of hydrogen in the materials (effected by changing process conditions). However, these conclusions were valid only for changing the hydrogen concentration in the films. Also, reducing the concentration of hydrogen may actually increase the number of "dangling bonds" in the amorphous silicon atomic network, thereby degrading initial quality of the material.
Initial defect density may affect the saturated defect density level of a particular amorphous silicon material. (The initial defect density is the density of defects inherently present in the material immediately after annealing). However, the relationship between these two defect densities is unclear and there is considerable disagreement among persons skilled in the art as to what the currently limited experimental findings mean. Aljishi et al., in Fritzche (ed.), Amorphous Silicon and Related Materials, p. 887, World Scientific, Singapore, 1988), found that increasing the germanium concentration in amorphous silicon caused a corresponding increase in initial defect density. The effect of germanium on saturated defect density was not reported.
Hence, there remains a need to reduce the saturated defect density of amorphous silicon materials so as to improve the photostability of these materials.
There is also a need for amorphous silicon materials exhibiting a lower saturated defect density.
There is also a need for a photovoltaic cell comprised of an amorphous silicon material that is photostable.
There is also a need for a photovoltaic cell comprised of an amorphous silicon material that, when light-soaked, exhibits a saturated defect density that is not significantly different from the initial defect density of the material or, in any event, less than about 2.times.10.sup.16 cm.sup.-3.